The reason why it isn't both is because the data suggests prop 13 isn't really changing homeownership periods by very much.

Consider this idea of "sitting on some land because taxes are low" with the reality that this land can't be developed in a very lucrative way against the cost of developing said land. That math changes overnight if that land is now allowed to have a 100 story tower on top of it instead of a single story commercial building perhaps with maybe four tenants paying rent. Now you are sitting on an asset you could actually sell for an appreciable amount of money to someone who will bring the financing to build that 100 story tower, and you can now dump that cash into some investments that will make you more rich.

Really, most of the world that is in a housing crisis fits this pattern: high demand for housing, existing environment built to the limits of zoned capacity, local government unwilling to ease zoning so as to preserve the power to anoint land for instantaneous demand driven development. Most (all?) of these places also regularly reassess property tax.